MIL-STD-1553B defines the data communications protocol, data format and the electrical characteristics of a local area network (LAN) for aircraft or other military vehicles. Typically, such a LAN includes transformers, transceivers, encoders, converters and terminal hybrids interconnected by a pair of redundant data buses.
To the extent concerning the present invention, LAN's of the MIL-STD-1553B type consist of a system controller and a plurality of terminals, all of which are capable of receiving and transmitting data over the buses, only one of which is active at any given time. All data transfers are initiated by the system controller. A data transfer is initiated when the system controller places a command word on the bus. The terminal addressed by the command word is either commanded to receive a message (herein termed a "RECEIVE" transfer) or to transmit a message (a "TRANSMIT" transfer). The addressed terminal responds with a status word. In a "RECEIVE" transfer, the message follows the command word. In a "TRANSMIT" transfer, it follows the status word.
Messages must not exceed a predetermined maximum number of data words (32 data words in the case of MIL-STD-1553). Command words and status words contain a synchronization code decodable as positive sync, while data words have a code decodable as negative sync.
If a plurality of redundant buses are used in the system, a Manchester II bi-phase level data encoding format is preferably used, as this format can easily produce an indication of whether a given bus is currently active or idle.
The MIL-STD-1553-type data bus is designed to operate within a limited physical bus length which is sufficient for installations within an aircraft vehicle, or small building. It is sometimes desirable, however, to extend the bus to a remote location in order to test the system or temporarily provide it with additional capabilities. Because the remote facility is typically beyond the maximum reach of the 1553B data bus, a repeater becomes necessary.
A problem arises, however, from the fact that the typical MIL-STD-1553 type system has two mutually exclusive redundant buses but the remote facility often has only one. Consequently, interfacing between the local area system and the remote facility was previously quite difficult and required special programming. Also, the necessary bidirectional repeating capability previously required two repeaters (one for each direction) for proper matching of each remote terminal.